Optical disc and method and apparatus for making same

ABSTRACT

An optical disc comprises two facing disc units, at least one of which has a recording layer formed on its surface facing to the other disc unit. The disc units are bonded together by a layer of adhesive which may be a room-temperature curing thermosetting adhesive having a glass transition temperature higher than the upper limit of the range of temperatures of environments in which the disc may be used. Alternatively, the adhesive may be a room-temperature curing two-pack epoxy adhesive which comprises a bisphenol epoxy resin base agent and a modified aliphatic polyamine curing agent, and which has a viscosity of 100-1000 cps, a pot life of one hour or more, a cure shrinkage of 1.0% or less, a water absorption, after curing, of 0.2% or less, and a Shore hardness, after curing, of 80-90.

This is a divisional application of Ser. No. 07/496,422 filed Mar. 20,1990.

This invention relates to an optical disc, and, more particularly to anerasable optical disc, which may be used as, for example, an externaldevice of computers This invention relates also to a method andapparatus for manufacturing such optical discs.

BACKGROUND OF THE INVENTION

FIG. 1 shows a cross-section of an ordinary optical disc, designated as10. A recording layer 2 is formed on a transparent substrate 1 of resin,such as, for example, polycarbonate, and a protective film 3 is formedover the recording layer 2. The substrate 1, the layer 2 and the film 3form a disc unit 4. Two such disc units 4 are bonded to each other atthe exposed surfaces of the respective protective films 3 with anadhesive layer 5. Thus, the optical disc 10 is formed. Conventionally,in order to bond the disc units 4 together, a thermosetting adhesive, anultraviolet-curable adhesive, or a thermoplastic adhesive are used. Acorrosive component contained in such adhesives, however, tends todegrade the recording layers 2, and, in particular, when the waterabsorption of the adhesive layer 5 is high, the protective films 3 anathe recording layers 2 are oxidized, which lowers the reliability of thedisc 10. A moisture-curing adhesive, such as one-pack epoxy resinadhesive, may be used. However, when disc units having large areas arebonded with a thin layer of a moisture-curing adhesive, air or moisturehardly penetrates to reach center portions of the bonded disc units,resulting in incomplete curing of the adhesive. Furthermore bonding discunits with such an adhesive requires a long time and is difficult to do.When two-pack non-mixing type resins and microcapsule type resins areused, some components may remain uncured, which will erode the discs.Furthermore, such adhesives can provide insufficient adhesion. For suchreasons, two-pack non-mixing type and microcapsule type adhesives areconsidered unsuitable for bonding disc units.

Another problem which may be encountered when curing type adhesives areused is that contraction of the curing adhesive may cause distortion ofthe disc units. In particular, when a thermosetting adhesive is used,not only does contraction of the adhesive occurs when it cures, but alsoheat applied for curing the adhesive distorts the adhesive layer 5itself. Distortion of the adhesive layer 5 causes deformation or warpageof the disc units, and, therefore, resulting optical discs may have tobe rejected.

The use of a thermoplastic resin adhesive rather than a thermosettingresin as the adhesive layer 5 is disclosed in, for example, JapanesePatent Publication No. SHO 63-67258. In the invention disclosed in thispatent publication, a hot-melt type adhesive, which is thermoplastic, isused for the adhesive layer 5. A hot-melt adhesive which has been heatedand melted is applied over the protective film 3 of a first disc unit 4.Then, the other disc unit 4 is placed on the first disc unit 4 in such amanner that the protective film 3 of the other disc unit 4 comes intocontact with the adhesive which has been applied over the protectivefilm 3 of the said first disc unit 4. Then, the assembly is cooled toroom temperature so that the hot-melt adhesive cures to bond the twodisc units 4 to each other.

Since such a thermoplastic resin adhesive need not be heated for itscuring, distortion of the adhesive layer 5 is small, which, in turn, canadvantageously reduce warpage of the disc units 4. However, if theadhesive is applied non-uniformly, portions of the adhesive layer 5 mayswell from moisture which penetrates through the substrate 1, and thedisc units may crack or may separate from each other.

Usually, optical discs are used in different environments, from colddistricts to hot districts, and, accordingly, the temperature at whichdiscs are designed to perform desired functions (hereinafter thistemperature is referred to as usable environment temperature) ranges forexample, from -20° C. to 60° C. The inventors have conductedhumidity-resistance tests and temperature-humidity cycle tests onoptical discs comprising two disc units 4 bonded together with ahot-melt adhesive at the above-stated temperature range of from -20° C.to 60° C. at which discs may be used. They found that the recordinglayers 2 were readily peeled off, pin holes were produced and, when thenumber of test cycles increased, bit error rates increased abruptly.Therefore they concluded that such optical discs were not sufficientlyreliable.

One conventional technique for bonding two disc units 4 together is asfollows. A first disc unit 4 is positioned with the recording layer 2facing upward. An adhesive is applied over the protective film 3 on therecording layer 2 in generally concentric circles. A second disc unit 4is aligned with the first disc by means of a center shaft of a discmanufacturing apparatus, and the protective film 3 of the second discunit is brought into contact with the adhesive. The resultant assemblyis left as it is so that the adhesive is spread over the entire surfacesof the protective films due to the weight of the disc unit, and theadhesive is caused to cure to bond the two disc units together. In thisbonding technique, bubbles may be disadvantageously formed in theadhesive when it is applied over the protective film or when the seconddisc unit is brought into contact with the adhesive on the protectivefilm of the first disc unit, and the bubbles may remain after theadhesive cures.

one method for preventing such bubbles from being formed in the adhesivelayer is shown in Japanese Unexamined Patent Publication No. SHO61-50231. According to the method shown in this patent publication, anadhesive is applied over the entire surface of the protective film 3 byspin-coating, and the two disc units are bonded together with theircenter axes aligned with each other. According to another method whichis shown in Japanese Unexamined Patent Publication No. SHO 61-292244, anadhesive is applied over portions of the protective film 3 of one discunit, the other disc unit is placed over the first disc unit with thecenter axes of the two disc units aligned, and pressure is applied tobond the two disc units together with the adhesive spread over theentire surfaces of the protective films.

When an adhesive is applied over the entire surface of the protectivefilm 3 of one disc unit 4 and the other disc unit is bonded to the firstdisc unit, the adhesive may forced out into the center holes of the discunits and also around the bonded disc units, and, when the adhesivecures, burrs 7 may be formed on the periphery of the center hole 6 ofthe optical disc 10 and on the periphery of the disc 10, as shown inFIGS. 2(a) and 2(b). Burrs 7 formed within the center hole 6 could makethe disc 10 eccentric, and, accordingly, they should be completelyremoved. In order to deburr, a deburring device, such as one shown inJapanese Unexamined Patent Publication No. SHO 61-00534, may be used.However, the use of a deburring device will undesirably increase thenumber of manufacturing steps, which, in turn, increases themanufacturing costs. On the other hand, even when the adhesive isapplied over portions of the protective film, it may be forced out orooze out as in the case when the adhesive is applied over the entiresurface as stated above, or, sometimes, the distribution of the adhesivemay be non-uniform, so that the disc 10 may flutter.

In order to prevent the adhesive from oozing out into the center hole orout of the periphery of the disc, a precisely adjusted pressure must beapplied, which requires high-precision, expensive equipment. One of thesimplest techniques for bonding two disc units is the use of the weightof a disc unit itself with an adhesive placed between the two discunits. In this technique, however, if an adhesive having viscosity ofless than 100 cps is used, it may ooze out and form burrs 7 like theones shown in FIGS. 2(a) and 2(b). In contrast, if the viscosity of theadhesive is above 1000 cps, the adhesive may not spread over the entirespace between the two disc units 4, as shown in FIGS. 3(a) and 3(b).Even if the amount of the adhesive to be applied is precisely measured,oozing out of the adhesive as shown in FIGS. 2(a) and 2(b) or absence ofthe adhesive at some portions as shown in FIGS. 3(a) and 3(b) may occurif the adhesive is applied to disc units at inappropriate positions.

A reduced-pressure bonding apparatus as shown in FIG. 4 has beenconventionally used for bonding, with an adhesive, two disc unitswithout leaving bubbles in the adhesive layer. In FIG. 4, a vacuumchamber 11 houses mounts 14 and 15 coupled respectively to shafts 12 and13 which can move up and down. Disc units 4 with adhesive layers 16 and17 applied over the surfaces of protective films 3 of the respectivedisc units 4 are mounted on the mounts 14 and 15, respectively. A vacuumpump (not shown) is operated to reduce the pressure in the vacuumchamber 11 through an exhaust pipe 18 to a pressure of about 20 Torr orless. Then, the shafts 12 and 13 move the mounts 12 and 13 toward eachother for bonding the disc units 4 together. The adhesive may be curedunder a reduced pressure or under normal pressure, but the pressureunder which the bonding of disc units is carried out must be about 20Torr or below. When the pressure is higher than that, bubbles may formin the adhesive.

When the reduced-pressure bonding apparatus of FIG. 4 is used, theadhesive is applied to at least portions of the protective films 3 ofthe disc units 4, and the disc units 4 are bonded with the adhesivewhich is spread over the entire surfaces of the films 3 due toapplication of pressure. It is, therefore, necessary to controlprecisely the movement of the shafts 12 and 13 in order to prevent theadhesive from oozing out into the center hole or to the outer peripheryof the disc or from being non-uniformly distributed. Furthermore, it isnecessary to determine precisely the amount of adhesive to be appliedand also the position where the adhesive is to be applied. In addition,it is also necessary to maintain the pressure in the reduced-pressurebonding apparatus at about 20 Torr or below. If one wants to use thistype of apparatus for mass-production of optical discs, the size of theapparatus must be large.

Therefore, a first object of the present invention is to produce opticaldiscs which are free of warpage of the discs and peeling off of arecording layer of the discs when they are used at a usable environmenttemperature within a range of, for example, from -20° C. to 60° C.

A second object of the present invention is to produce, at low costs,highly reliable optical discs in which degradation of recording layersand deformation of disc units are eliminated by the use of such anadhesive that can reduce the number of discs to be rejected in the stepof bonding two disc units.

A third object of this invention is to provide an improved method ofmanufacturing optical discs, according to which, when two disc units arebonded together with an adhesive, the adhesive does not ooze out intothe center hole or to the outer periphery of a resulting disc, and,accordingly, a step for removing burrs can be eliminated. The resultingoptical discs are free of eccentricity and free of surface fluttering.

SUMMARY OF THE INVENTION

In a first example of an optical disc according to the presentinvention, the optical disc comprises two disc units facing each other,at least one of which has a recording layer formed on the surface facingthe other disc unit. The two disc units are bonded together by means ofan adhesive layer which is disposed between the facing surfaces andwhich comprises a thermosetting resin that has a glass transitiontemperature higher than the highest usable environment temperature ofthe optical disc and that cures at room temperature.

In a second example of an optical disc according to the presentinvention, two disc units, each comprising a transparent substrate and arecording layer formed on one surface of the substrate, are disposedwith the recording layers facing each other, and the disc units arebonded together by means of an adhesive layer interposed between therecording layers. The adhesive layer is formed by curing aroom-temperature curable two-pack epoxy adhesive which comprises abisphenol epoxy resin as a base agent and a modified aliphatic polyamineas a curing agent, and which has a viscosity of 100-1000 cps, a pot lifeof one hour or more, and a cure shrinkage of the adhesive of 1.0% orless. The adhesive, when cured, has a water absorption of 0.2% or less,and has a Shore hardness of 80-90 (D scale).

For example, the bisphenol epoxy resin of A-type or F-type may beadvantageously used. The modified aliphatic polyamine having a viscosityof, for example, 100 cps or less may be advantageously used.

According to one aspect of the method of manufacturing optical discs ofthe present invention, an adhesive, which forms the adhesive layer ofthe above-described first and second examples, is applied to a surfaceof one of two disc units along the circumference of one or more circlesabout the center of the disc unit having a radius of from 0.5a to 0.85a,where a is the radius of the disc unit, and then the two disc units arebonded together.

A second example of the method of manufacturing optical discs of thepresent invention comprises the steps of: applying an adhesive to thesurface of the first of two disc units to be bonded together which facesthe second disc unit; placing the second disc unit slantwise with oneend of a diameter of the second disc unit abutting one end of a diameterof the first disc unit; supporting the first and second disc units bymeans of a center shaft of a base; holding the second disc unit at theother end of the said diameter, while maintaining the second discslanting with respect to the first disc unit, by holding means extendingfrom driving means mounted on an axis which is substantially in parallelwith the line connecting the respective other ends of the diameters ofthe first and second disc units; moving the driving means downwardly tobring the two disc units into contact with each other in such a mannerthat portions of the disc units near to the one ends of the diametersfirst contact with each other and portions of the disc units near to theother ends contact with each other last; and curing the adhesive to bondthe disc units together.

In each of the examples of the method of manufacturing optical discsdescribed above, the adhesive used in the previously described first andsecond examples of optical discs may be used.

An optical disc manufacturing apparatus of the present inventionincludes a center shaft mounted on a base for supporting two disc units,holding means for holding one end of a diameter of one of the two discunits in such a manner that the one disc unit is slanting with respectto the other disc unit, driving means supporting the holding means andmovable up and down at an adjustable speed, and angle adjusting meansfor adjusting the slanting angle of the driving means in accordance withthe angle of the one disc unit relative to the other disc unit.

BRIEF DESCRIPTION OF THE INVENTION

In the accompanying drawings,

FIG. 1 is a cross-sectional view of a portion of a common optical disc;

FIGS. 2(a) and 2(b) are plan and cross-sectional views, respectively, ofan optical disc comprising two disc units bonded with a low-viscosityadhesive according to a conventional technique;

FIGS. 3(a) and 3(b) are plan and cross-sectional views, respectively, ofan optical disc comprising two disc units bonded with a high-viscosityadhesive according to a conventional technique;

FIG. 4 is a cross-sectional view of a conventional reduced-pressurebonding apparatus for bonding two disc units;

FIG. 5 is a cross-sectional view of a portion of an optical discaccording to one embodiment of the present invention;

FIG. 6 shows expansion coefficients of an adhesive layer of the presentinvention and a substrate, together with an expansion coefficient of aconventional adhesive layer, in relation to temperature;

FIG. 7 shows bit error-rates of optical discs which are manufacturedusing adhesives of the present invention and conventional adhesives, toillustrate how bit error rates change with the number of cycles incomposite temperature/humidity cyclic tests conducted according toJIS-C5024, Procedure I;

FIG. 8 shows how much corrosion occurs in optical discs of the presentinvention and in conventional optical discs;

FIG. 9 shows the ratio of a corroded area of an optical disc to thetotal area of the disc for adhesives which use curing agents ofdifferent viscosities;

FIG. 10 shows the relationship between the number of corrosions and thewater absorption of optical discs of the present invention andconventional optical discs, in which the water absorptions were measuredafter boiling the discs in water for one hour after the adhesive hascured:

FIG. 11 shows a maximum tilt angle and a maximum amount of increase ofbirefringence caused by bonding, of each of optical discs of the presentinvention and conventional optical discs employing layers of adhesivesof different curing types;

FIG. 12 shows maximum tilt angles showing maximum warpage caused by cureshrinkage of curing agents, and maximum amounts of increase ofbirefringence caused by bonding, of optical discs of the presentinvention and conventional optical discs;

FIG. 13 shows the number of swells produced in optical discs of thepresent invention and conventional discs with adhesives of differentcuring types after thirty (30) cycles of JIS-C5024 Procedure I compositetemperature/humidity cyclic tests have been performed;

FIG. 14 shows the number of swells in optical discs of the presentinvention and conventional optical discs, and their breaking strengthagainst dropping, in relation to the hardness of various adhesives ofdifferent curing types after thirty (30) cycles of JIS-C5024 Procedure Icomposite temperature/humidity cyclic tests have been performed;

FIG. 15 shows how much area is left with no adhesive applied or how farthe applied adhesive extends out of the periphery of disc center holesand out of the outer periphery of optical discs of the present inventionand conventional optical discs, for various viscosities of adhesivesapplied with no pressure exerted to disc units;

FIG. 16 shows schematically the structure of apparatus suitable formanufacturing optical discs of the present invention;

FIG. 17 shows changes, with time, in C/N ratio and bit error rate ofoptical discs comprising two disc units bonded together, which can beindexes of operation reliability of discs; and

FIG. 18 shows another example of apparatus which may be used tomanufacture optical discs of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail with reference to theaccompanying drawings.

As shown in FIG. 5, an optical disc according to the present inventionmay be manufactured in the following manner. A layer 22 of dielectricmaterial, such as silicon nitride (SiN_(x)), is formed over a surface,in which grooves may be formed, of a transparent substrate 21 ofsynthetic resin, such as polycarbonate, having a glass transitiontemperature of, for example, 130° C. Then, over this dielectric layer22, a recording layer 23 is formed. The recording layer 23 may comprisean amorphous magnetic material having perpendicular magnetic anisotropy,such as terbium-iron-cobalt (Tb-Fe-Co). A protective film 24 of, forexample, silicon nitride (SiN_(x)) is formed over the recording layer 23to complete a disc unit 20. Two such disc units 20 are bonded together,with their respective protective films 24 facing each other, with anadhesive layer 25 interposed between them. The basic structure of thisoptical disc is substantially the same as that of conventional ones. Theoptical disc of the present invention is characterized by the materialof the adhesive layer 25.

According to the present invention, the adhesive layer 25 comprises athermosetting adhesive which has a glass transition temperature higherthan the upper limit of the usable environment temperature range of, forexample, from -20° C. to 60° C., of the disc, and which can cure at roomtemperature. An example of adhesive usable in the present invention is atwo-pack room-temperature curable epoxy adhesive which comprises abisphenol epoxy resin, as a base, and a modified aliphatic polyamine, asa curing agent, has a viscosity of from 100 to 1000 cps, and has a potlife of one hour or longer. This adhesive can cure at room temperatureand has a glass transition temperature of about 70° C., and, therefore,it can satisfy the above-described conditions. The above-describedtwo-pack epoxy adhesive has to have a cure shrinkage of 1.0% or less, awater absorption of 0.2% or less, and a Shore hardness of from 80 to 90(D scale), after it cures.

Another example of usable adhesive is a two-pack room-temperaturecurable epoxy adhesive which comprises a bisphenol epoxy resin, as itsbase, and a modified aliphatic polyamine, as the curing agent, having aviscosity of 100 cps or less at room temperature. This adhesive has aviscosity of 100-1000 cps and a pot life of one hour or longer, and canbe cured at room temperature. Like the first described example, thisadhesive should have a cure shrinkage of 1.0% or less, a waterabsorption of 0.2% or less, and a Shore hardness of 80-90 (D scale).

Bisphenol epoxy resins usable as the base agent of the adhesives of thepresent invention are a bisphenol A epoxy resin (commercially availableas TB2022 (trade name) from Three Bond Co., Ltd. Hachioji-shi, Tokyo,Japan), and a bisphenol F epoxy resin (commercially available as TB2023from Three Bond Co., Ltd.). Experiments have revealed that two-packepoxy adhesives other than the above-described ones and one-pack epoxyadhesives cause the recording layers to be oxidized and degraded due tocorrosive components contained in such adhesives.

In some applications, adhesives to be used for manufacturing opticaldiscs of the present invention may be selected from the viewpoint ofglass transition temperature and warpage of substrates or disc units.

EXAMPLE 1

The adhesive layer 25 comprises a two-pack epoxy adhesive comprising amixture of a bisphenol A epoxy base agent (available as TB2022 fromThree Bond Co., Ltd.) and a modified aliphatic polyamine curing agent(available as TB2131D from Three Bond Co. Ltd.) which are mixed in aratio of 3:1. The mixture is then deaerated. This adhesive can cure atroom temperature, and it has a glass transition temperature of about 70°C. As indicated by a solid line in FIG. 6, the adhesive has a thermalexpansion coefficient substantially the same as that of thepolycarbonate substrate 21 indicated by a dash-and-dot line, in thetemperature range below the glass transition temperature (T_(g2)) of theadhesive. The polycarbonate substrate 21 has a glass transitiontemperature T_(g1).

Since the adhesive of Example 1 could cure and form the adhesive layer25 at room temperature, distortion of the layer produced when theadhesive cures was small, and the amount of warpage of the substrates 21measured in terms of tilt angle after the two disc units were bondedtogether was 1.5 mrad, which was small relative to the values forComparisons 1, 2 and 3 described later. Incidentally, a tilt angle isthe angle of tilt of the normal to a surface of a disc with respect tothe axis of rotation of the disc and can be a measure of the amount ofwarpage of the disc.

Optical discs having a similar structure to the disc of Comparison 1 andcomprising two disc units 20 bonded together by the above-describedadhesive were placed in a thermo-hygrostat whose temperature andrelative humidity (RH) were held at 60° C. and 90%, respectively, for100 hours to test their humidity resistance, and the tilt angle wasmeasured. The measured tilt angle was 1.6 mrad, which was only slightlydifferent from the tilt angle measured before the test. Furthermore,this value was smaller than the tilt angles of Comparisons 1, 2 and 3,described later, measured immediately after the bonding. The opticaldiscs of Example 1 were also subjected to composite temperature/humiditycyclic tests according to JIS-C5024 Procedure I (in which thetemperature was between -10° C. and 65° C. and thirty test cycles wereperformed). (As for the JIS-C5024, Procedure I compositetemperature/humidity cyclic test, see IEC 68-2-38 Basic EnvironmentalTesting Procedures, Part 2.) After the tests, the recordingcharacteristics of the optical discs were measured. As indicated by asolid line a in FIG. 7, substantially no increase in bit error rate fromthe initial value was observed, which means that the optical discs ofExample 1 are highly reliable.

EXAMPLE 2

The adhesive layer 25 of Example 2 comprises a mixture of the samebisphenol A epoxy base agent as in Example 1 (available as TB2022 fromThree Bond Co., Ltd.) and a modified alicyclic polyamine curing agent ina ratio of 2:1. This adhesive has a glass transition temperature of 90°C. and cures at room temperature.

The tilt angles of optical discs measured immediately after two discunits 20 were bonded together with the adhesive of Example 2 was 1.4mrad, which was small relative to that of the later-mentionedComparisons 1-3. The tilt angles indicative of the amounts of warpagemeasured after the discs were subjected to humidity resistance tests was1.8 mrad which was also smaller than the tilt angles of the discs ofComparisons 1-3 measured immediately after the bonding. Furthermore, therecording characteristics of the optical discs employing the adhesive ofExample 2 measured after temperature/humidity cycle tests were alsogood.

EXAMPLE 3

The adhesive layer 25 used in this example is an acrylicultraviolet-curing resin (commercially available, as TB3033, from ThreeBond Co., Ltd.) which has a glass transition temperature of 80° C. andcures at room temperature.

The tilt angle of the disc measured immediately after two disc unitswere bonded together with the adhesive of Example 3 was 1.7 mrad, whichwas small relative to those of Comparisons 1-3. The tilt angle of thediscs measured after humidity resistance tests was 1.9 mrad which wasalso smaller than those of Comparisons 1-3 measured immediately afterthe bonding. Further, the recording characteristics of the optical discsemploying the adhesive of Example 3 measured after temperature/humiditycyclic tests were also good.

EXAMPLE 4

The adhesive forming the adhesive layer 25 of this example is a two-packepoxy adhesive which comprises a base agent (TB2022C available fromThree Bond Co., Ltd.) comprising a bisphenol A epoxy resin to which areactive diluent having an epoxy group is added, and a modifiedaliphatic polyamine curing agent (TB2131D available also from Three BondCo., Ltd.), mixed in a ratio of 3:1. The adhesive of Example 4 has aglass transition temperature of 60° C. and cures at room temperature.

The tilt angle of optical discs employing the adhesive of Example 4measured after two disc units were bonded was 2.0 mrad; This is as smallas that of Comparison 1 which was the smallest in the three Comparisons.The amount of warpage of the optical discs of this Example slightlyincreased after they were subjected to humidity resistance tests, butsuch an increase was practically negligible. A-dash-and-dot line c inFIG. 7 indicates the bit error rates measured for the optical discs ofExample 4 after respective cycles of temperature/humidity cyclic tests.As is seen, the bit error rate remained substantially the same as theinitial value when the discs were subjected to twenty or less testcycles. This means that the optical discs of Example 4 have sufficientdurability and reliability. The bit error rate begins to increase moreor less when the discs are subjected to thirty cycles of the test, butis still practically negligible.

In order to demonstrate the advantageous features of the adhesives usedin accordance with the present invention, various characteristics ofoptical discs using conventionally used adhesives are describedhereinafter.

COMPARISON 1

In Comparison 1, the adhesive used for forming an optical disc adhesivelayer 25 is a hot-melt adhesive which has a glass transition temperatureof 5° C. and cures at room temperature.

The tilt angle of discs of Comparison 1 measured after two disc unitswere bonded together was 2.0 mrad, which was small enough. However, thetilt angle of substrates 21 increased considerably to 21.5 mrad, afterthe discs were subjected to humidity resistance tests in which the discswere placed within a thermo-hygrostat of 60° C. and 90% RH for 100hours. It is understood that the discs were not usable. The discs ofComparison 1 were also subjected to composite temperature/humiditycyclic tests according to JIS-C5024 Procedure I (temperature rangingfrom -10° C. to 65° C., the number of test cycles being 30 cycles). Asis shown in FIG. 7, by a broken line b, the bit error rates of the discsafter the cycle tests were relatively high, and, in particular, when thediscs were subjected to 20 or more cycles of tests, the bit error rateincreased sharply, which means that the discs of Comparison 1 are notsufficiently reliable.

COMPARISON 2

The adhesive of the adhesive layer 25 of optical discs of Comparison 2comprises a mixture of a bisphenol A epoxy resin (Epicoat 828commercially available from Shell Chemical, Co.), as a base agent, and acuring agent comprising polythiol. They are mixed in a ratio of 1:1.This adhesive has a glass transition temperature of 50° C. and cures atroom temperature.

The tilt angle of the optical discs of Comparison 2 measured after twodisc units were bonded together was 2.4 mrad, which was small anddesirable. However, the tilt angle of substrates 21 increased to 17.2mrad after the discs were subjected to humidity resistance tests similarto those conducted for Comparison 1, and the bit error rate rapidlyincreased after the discs were subjected to temperature/humidity cyclictests. That is, the optical discs of Comparison 2 were neither durablenor reliable.

COMPARISON 3

The adhesive of the adhesive layer 25 of optical discs comprises amixture of a base agent which is a bisphenol A epoxy resin (Epicoat 828)and an imidazole compound. They are mixed in a ratio of 100:2. Thisadhesive is heated to 80° C. for curing. The glass transitiontemperature is 130° C.

The tilt angle of the discs of Comparison 3 measured after two discunits were bonded together with the adhesive was 10.3 mrad, which is toolarge for practically usable optical discs.

Data concerning Examples 1-4 of the present invention and Comparisons1-3 are shown in the following Table.

                                      TABLE                                       __________________________________________________________________________                              Tilt Angle After                                           Glass Transition                                                                        Tilt Angle After                                                                       Subjecting To 60° C.,                                                              Recording Characteristics                                                                     Curing                         Temperature (°C.)                                                                Bonding (mrad)                                                                         90% RH for 100 Hours                                                                      After Temp./Hum. Cyclic                                                                       Temperature             __________________________________________________________________________    Example 1                                                                            70        1.5 (good)                                                                             1.6 (good)  (good)          room temperature        Example 2                                                                            90        1.4 (good)                                                                             1.8 (good)  (good)          room temperature        Example 3                                                                            80        1.7 (good)                                                                             1.9 (good)  (good)          room temperature        Example 4                                                                            60        2.0 (good)                                                                             2.6 (rather good)                                                                         (rather good)   room temperature        Comparison 1                                                                          5        2.0 (good)                                                                             21.5                                                                              (bad)   (bad)           room temperature        Comparison 2                                                                         50        2.4 (good)                                                                             17.2                                                                              (bad)   (bad)           room temperature        Comparison 3                                                                         130       10.3                                                                              (bad)                                                                              --          --              80° C.           __________________________________________________________________________

As is seen from this Table, all of the adhesives of the presentinvention forming the adhesive layers 25 disposed between two disc unitsof optical discs have a glass transition temperature (T_(g2)) higherthan the upper limit (60° C.) of the usable environment temperaturerange T_(d) and are curable at room temperature. Further, when they areused at a temperature within the usable environment temperature rangeT_(d), the warpage of the substrates 21 is small and has sufficientdurability and reliability.

The reasons why such good results can be obtained are considered to beas follows. As is shown by the, temperature-linear expansion coefficientcharacteristic in FIG. 6, the glass transition higher than the upperlimit (60° C.) of the usable environment temperature range (T_(d)), and,therefore, the linear expansion coefficients of the substrates 21 andthe adhesive layers 25 are substantially equal to each other inenvironments where the discs are normally used. Accordingly, even iflarge temperature changes occur, the adhesive layers are not distortedand, accordingly, neither warping of the discs nor peeling off of therecording layers 23 occurs. When an adhesive such as the one ofComparison 1 which has a glass transition temperature T_(g3) that islower than the upper limit (60° C.) of the usable environmenttemperature range T_(d) is used, the linear expansion coefficient of theadhesive become greatly different from that of the substrates even inthe temperature range T_(d), which may cause discs to warp or may causethe recording layers to be peeled off.

Because water absorption of adhesives of resins is very high attemperatures above the glass transition temperature, the adhesives ofComparisons 1 and 2, whose glass transition temperatures are low, absorbmoisture even within the temperature range T_(d). Water absorbed in theadhesives accelerates warping and corrosion of optical discs, whichcauses bit error rates to greatly increase. These are the reasons whythe optical discs with the adhesives having a glass transitiontemperature higher than the upper limit (60° C.) of the usableenvironment temperature range can produce good results.

The adhesive of Comparison 3 has a high glass transition temperature of130° C. However, since this adhesive needs to be heated to cure, theadhesive layer 25 is distorted while it is curing and, therefore,optical discs using this adhesive greatly warp. Thus, adhesives like theone of Comparison 3 are not suitable for use in optical discs.

In some applications, adhesives may be chosen from the viewpoint ofcorrosion, warpage and the number of swells of discs caused by adhesivesused, distribution of applied adhesives, etc.

FIG. 8 shows corrosion of optical discs according to some embodiments ofthe present invention in comparison with corrosion of conventionaloptical discs. The percentage of corroded area to the total disc area isthe ordinate. The values were measured after the respective discs werestored in an atmosphere the temperature and relative humidity of whichwere 60° C. and 90%, respectively. The discs were placed in theatmosphere for 1,000 hours. Discs A₁ -A₅ use Adhesives each comprising amixture of a bisphenol A epoxy resin (TB2022 available from Three BondCo., Ltd.) and modified aliphatic polyamine (TB2131D available fromThree Bond Co., Ltd.), but the viscosities of the curing agents in therespective adhesives are different. (For instance, the viscosity of thecuring agent of A₄ is 110 CPS, and that of A₅ is 2800 cps.) Discs B₁ -B₃use adhesives each comprising a mixture of the bisphenol A epoxy resinand a modified polyamideamine (polyamide accelerated by polyamine)curing agent, but the viscosities of the curing agents are different.Discs C₁ -C₃ use adhesives each comprising a mixture of the bisphenol Aepoxy resin and a polythiol curing agent, but the viscosities of thecuring agents are different. Further, discs D₁ -D₃ use differentthermoplastic adhesives for optical disc use.

FIG. 9 shows the percentage of corroded areas of respective opticaldiscs in which adhesives comprising a bisphenol A epoxy resin (TB2022available from Three Bond Co., Ltd.) and modified aliphatic polyamine(TB2131D available from Three Bond Co., Ltd.) are used. The viscositiesof the modified aliphatic polyamine in the respective adhesives aredifferent. The ordinate is the percentage of corroded area, and theabscissa is the viscosity of the modified aliphatic polyamine (in cps at25° C.). Except for the adhesives used, the optical discs shown in FIG.9 have been made under similar conditions. As shown in FIGS. 8 and 9,the combinations of bisphenol A epoxy and modified aliphatic polyamineprovide less corrosion, and those in which the viscosity of the curingagent is 100 cps (at 25° C.) or less provide much less corrosion. Thedisc D₁ (FIG. 8) has a relatively low percentage of corroded area, butit tends to have a large number of swells, as will be described later,and, therefore, is not suitable for use as an optical disc.

FIG. 10 shows the number of corroded regions in optical discs which useadhesives comprising a bisphenol A epoxy resin (TB2022 available fromThree Bond Co., Ltd.) and modified aliphatic polyamine (TB2131Davailable from Three Bond Co., Ltd,.), in relation to water absorptions(%) of the adhesives after they cure. The adhesives used have differentwater absorptions after they have cured, which are determined afterboiling them for one hour in water. The number of corroded regions weremeasured after storing the discs at 60° C. and at 90% RH for 1000 hours.The number of corroded regions is the ordinate, while the waterabsorption in percent (%) is the abscissa. As shown in FIG. 10, discswith layers of adhesives of which the water absorption after curing is0.2% or less have no corrosion. Thus, adhesives to be used for opticaldiscs should have a water absorption after curing of 0.2% or less.

FIG. 11 shows maximum tilt angles (solid line) and maximum increases inbirefringence caused by bonding (broken line), of optical discs in whichadhesives of different curing types are used. In FIG. 11, (A) is a discwhich uses a room-temperature curing adhesive, such as one comprising acombination of a bisphenol A epoxy resin (TB2022 available from ThreeBond Co., Ltd.) and modified aliphatic polyamine (TB2131D available fromThree Bond Co., Ltd.), (X) is a disc using a conventional adhesive whichcures when heated to 30° C., (Y) is a disc using a conventional adhesivewhich cures when heated to 80° C., and (Z) is a disc using aconventional ultraviolet curing adhesive.

FIG. 12 shows maximum tilt angles (solid line) and maximum increases inbirefringence caused by bonding (broken line), on the ordinate, inrelation to cure shrinkages, on the abscissa, of the optical discs usingadhesives of the above-stated room-temperature curing type.

The data show that, whether the adhesive used is of heat curing typewhich cures at a temperature above 30° C., of the ultraviolet curingtype, or of the room temperature curing type, if the cure shrinkage ofthe adhesive is above 1.0%, distortion after curing is too large. Thisproduces disc deformation and/or increase of warpage and birefringence.This means that the adhesives for bonding disc units together must becurable at room temperature and have a cure shrinkage of 1.0% or less.

FIG. 13 shows the number of swells produced in optical discs withadhesives of various curing types used for bonding disc units when thediscs are subjected to 30 cycles of composite temperature/humiditycyclic tests conducted according to JIS-C5024 Procedure I. In FIG. 13,the ordinate indicates the number of swells and the abscissa indicatesoptical discs which employ adhesives of various curing types for theiradhesive layers. A₁ -A₄ represent discs using room-temperature curingepoxy resins which comprise, for example, a bisphenol A epoxy resin(TB2022 available from Three Bond Co., Ltd.) and modified aliphaticpolyamine (TB2131D available from Three Bond Co., Ltd.) and have a Shorehardness of 85 (D scale). D₁ -D₄ represent discs using thermoplasticadhesives applied by means of a roller. It is seen from FIG. 13 thatnon-uniform distribution of roller-applied thermoplastic adhesives maypermit moisture to penetrate through the substrates and stay at portionsbetween the substrates and the adhesive layer, which causes swellings tobe formed in the protective films and recording layers. Such swells maycause the protective films and/or recording layers to be peeled off orto be cracked. This shows thai the adhesive layers must be uniform andalso have hardness of a given value or more.

Next, adhesives of different hardness values were tested. FIG. 14 showsthe number of swells (solid line) formed in optical discs and also dropstrength (broken line) of the discs after they are subjected to 30cycles of temperature-humidity cycle tests, in relation to the Shorehardness of the adhesives. The tested discs include layers of differenthardness adhesives which essentially comprises a room-temperature curingexpoxy resin comprising, for example, a bisphenol A epoxy resin (TB2022available from Three Bond Co., Ltd.) and modified aliphatic polyamine(TB2131D available from Three Bond Co., Ltd.). The drop strength isexpressed in comparison with the value, which is 100, of a disc using anadhesive having a Shore hardness of 85 (D scale). As shown in FIG. 14,the number of swells in optical discs steeply decreases substantially to0 when the Shore hardness (D scale) is above 80. However, drop tests fordetermining the breaking strength of discs revealed that discs usingadhesives having a Shore hardness of 90 or below have substantially thesame drop strength, but when a Shore hardness of the adhesive used indiscs is above 90, the discs are easily broken when dropped.Accordingly, it is necessary that the adhesives have a Shore hardness (Dscale) of from 80 to 90.

Curing-type adhesives essentially comprising a room-temperature curingepoxy resin which comprises, for example, a bisphenol A epoxy resin(TB2022 available from Three Bond Co., Ltd.) and modified aliphaticpolyamine (TB2131D available from Three Bond Co., Ltd.), and havingdifferent viscosities, were applied over disc units with no pressureapplied to the adhesives. FIG. 15 shows a ratio (%) of area of portionsleft uncoated to the total area of discs (solid line) or a ratio (t) ofthe length of protrusion of adhesives out of the peripheries of thediscs (broken line), in relation to the viscosity of the adhesives.Along the abscissa, the viscosity of the adhesives when they are appliedover disc units is indicated in cps at 25° C. Along the ordinate, thepercentage of the area of uncoated portions arid the percentage of thelength of adhesive protrusions are indicated. From FIG. 15, it is seenthat the viscosity of adhesives to be used for bonding disc units mustbe from 100 cps to 1000 cps in order to form uniform adhesive layers. Inthe bonding step, when the viscosity of adhesives upon application todisc units is above 1000 cps, the percentage of portions left uncoatedincreases rapidly. On the other hand, if the viscosity is less than 100cps, or if pressure is applied to adhesives so as to leave no uncoatedportions, the adhesives will protrude from the inner periphery of thedisc center hole or the outer periphery of the disc, and, therefore, themechanical characteristics of discs are adversely affected, or an extrastep for removing such protruding adhesive becomes necessary.

Further, in order to form a uniform adhesive layer free of bubbles, thepot life of adhesives should be more than one hour, since mixing,deaeration or debubble, coating and bonding steps require at least onehour in total.

Now, bearing in mind the above results, the present invention isdescribed in greater detail by means of further examples.

EXAMPLE 5

A two-pack epoxy adhesive was prepared by mixing, at room temperature, abisphenol A epoxy resin base agent (TB2022 available from Three BondCo., Ltd.) having a viscosity of 13,000 cps (at 25° C.) and a modifiedaliphatic polyamine curing agent (TB2131D available from Three Bond Co.,Ltd.) having a viscosity of 10 cps, in a ratio of 3:1. The mixture wasdeaerated. The resultant adhesive had a vicosity of 400 cps (at 25° C.),a pot life of 5 hours, a water absorption after curing of 0.1% (afterboiling in water for one hour), a cure shrinkage of 0.05%, and a Shorehardness of 85 (D scale).

As shown in FIG. 16, in a normal pressure environment, one of highlycorrosion-resistant, Tb-Fe-Co disc units 201 having a diameter of, forexample, 130 mm was placed horizontally on a support 30 secured to acenter shaft 31 which extended from a base 29, with the center hole inthe disc unit 201 being fitted over the shaft 31. 0.5 g of the adhesiveof Example 5 was applied in a circle having a radius of 40 mm on thedisc unit 201. This quantity of the adhesive was to provide a thicknessof from 20 to 70 microns of the adhesive layer when it cured. Then, thesecond disc unit 202 to be bonded to the first disc unit 201 was fittedover the center shaft 31, and only one point on the peripheral edge ofthe second disc unit 202 was brought into contact with one point on theperiphery of the first disc unit 201. Thus, the second disc unit 202 washeld slanting relative to the first unit 201. Preferably, an adhesive 32is applied along the circumference of a circle having a radius of from0.5a to 0.85a from the center of the disc unit 201, where a is theradius of the disc unit 201. In the example, the adhesive was appliedalong the circumference of a circle having a radius of about 0.6a fromthe center.

A holding mechanism for the second disc unit 202 comprises a base 61 ofwhich the slanting angle is adjusted by angle adjusting means 33, ashaft 60 attached to the base 61, drive means 62 movable up and downalong the shaft 60, holding means 63 held by the drive means 62, and anextension 64 extending from the holding means 63. The slanting angle ofthe base 61 is adjusted so that the shaft 60 becomes substantiallyparallel with the line which connects points on the peripheral edges ofthe first and second disc units 201, 202 diametrically opposite to theaforementioned points which are in contact with each other. The tip endof the extension 64 is in engagement with the aforementioneddiametrically opposite point on the outer periphery of the second disc202 to hold the unit 202 slanted as shown. The drive mean& 62 is thenlowered along the shaft 60 at a rate of, for example, about 1 mm/sec. soas to slowly place the second disc unit 202 on the first disc unit 201.When the second disc unit 202 has been lowered, the tip end of theextension 64 is still in engagement with the second disc unit 202,although the engagement is slight. Then, the holding means 63 isactuated to retract the extension 64 to disengage from the second discunit 202. Then, the weight of the disc unit 202 causes the adhesive tospread over the entire space between the two disc units 201 and 202,while causing no oozing of the adhesive from the inner periphery of thecenter hole or from the outer periphery of the disc and also leaving noportions uncoated with the adhesive.

The disc was left at room temperature for 24 hours to cure the adhesive.The maximum tilt angle of the disc measured was 0.9 mrad, and themaximum amount of increase of the birefringence was 2.1 nm. These valuesshow that the disc is satisfactory. This disc was left in a 60° C., 90%RH atmosphere for 3,000 hours to see how much the disc was corroded.FIG. 17 shows the results. Because no corrosion was produced, the C/N(carrier/noise) ratio did not decrease, or the B.E.R. (bit error rate)did not increase. This means that the optical disc is highly reliable.

Further, the disc was subjected to severe temperature/humidity cyclictests to determine how the protective films and the recording layersstood up. Even after the disc was subjected to 30 cycles of the tests,no swell was-formed, neither the protective films nor the recordinglayers peeled off, and neither the C/N nor the B.E.R. changed.

EXAMPLE 6

The same process and the same apparatus as used in Example 5 wereemployed for making an optical disc, but, in place of the bisphenol Aepoxy base agent (TB2022 of Three Bond Co., Ltd.) used in Example 5, abisphenol F epoxy (TB2023 available from Three Bond Co., Ltd.) was used.Tests similar to the ones conducted for the optical disc of Examples 1-5were conducted, and similar good results were obtained.

As described above, according to the present invention, in order to bondtogether two disc units, each including a transparent substrate ofsynthetic resin and a recording layer, with the recording layersfacing-each other, a layer of adhesive is disposed between the facingrecording layers, which adhesive is a room-temperature curing two-packepoxy adhesive comprising a base agent of bisphenol epoxy resin and acuring agent of modified aliphatic polyamine. The adhesive has aviscosity of 100-1000 cps, and a pot life of more than one hour. Aftercuring, the adhesive layer has a cure shrinkage of 1.0% of less, a waterabsorption of 0.2% or less, and a Shore hardness of 80-90 (D scale). Theuse of such an adhesive can avoid degradation of the recording layers,deformation of the resultant optical discs, and formation of swells.Thus, products to be rejected due to undesirable bonding are reduced,and highly reliable, high quality optical discs can be manufactured.

According to an aspect of the present invention, as the curing agent ofthe above-described adhesive, modified aliphatic polyamine which has aviscosity of 100 cps or less is used. The use of such a curing agent canprovide optical discs having, in addition to the above-describedadvantages, higher reliability.

For manufacturing the optical disc of the present invention, anapparatus as shown in FIG. 18 may be used, as well. In FIG. 18, a base29, a center shaft 31 for supporting disc units 201, 202, and a support30 are similar to the corresponding components shown in FIG. 16, and theposition on the disc unit 201 where the adhesive is to be applied is thesame as in FIG. 16. In the apparatus of FIG. 18, rotation means 42 ismounted on vertical drive means 41. An arm 44 pivots about a pivot 43 onthe rotation means 42. Holding means, such as a suction means 45 isattached to the tip end of the arm 44 to hold the disc unit 202 slantingas shown.

Under a normal pressure condition, the first disc unit 201 having adiameter of, for example, 30 mm is placed horizontal on the support 30with the center shaft 31 extending through the center hole in the discunit 201. Then, 0.5 g of the adhesive 32 is applied onto the disc unit201 along the circumference of a circle having a diameter of about 40mm. The second disc unit 202 is then place in the apparatus, with thecenter shaft 31 extending through the center hole of the disc unit 202.In this case, one point on the outer periphery of the disc unit 202 isbrought into contact with the corresponding point of the first disc unit201. The second disc unit 202 is held by the suction means 45 at theportion diametrically opposite to that one point. Thus, the second discunit 202 is held slanting as shown. In this case, the vertical positionof the drive means 41 and the angle of the arm 44 are so adjusted thatthe arm 44 is in parallel with the surface of the second disc unit 202.

Then, the rotation means 42 is activated so as to cause the arm 44 topivot in such a manner that the tip end of the arm 44 is lowered at arate of, for example, 1 mm/sec. Thus, the second disc unit 202 is placedon the first disc unit 201. Seeing that the adhesive is spread over theentire space between the two disc units by the weight of the second discunit 202, the adhesive 32 is cured at room temperature. In place of theillustrated jack-type device, any other types of drive means 41 can beused.

What is claimed is:
 1. A method of manufacturing an optical disc, saidoptical disc including a first and a second disc unit, at least one ofsaid first and said second disc units including a recording layer, saidmethod comprising the steps of:supporting said first disc unit generallyhorizontally on a shaft; applying an adhesive onto said first disc unit;said adhesive being a thermosetting resin that cures at room temperatureand has a glass transition temperature higher than 60° Celsius; fittingsaid second disc unit on said shaft out of contact with said first discunit; bringing a first point on an outer periphery of said second discunit into contact with a first point on an outer periphery of said firstdisc unit, with said second disc unit slanting relative to said firstdisc unit; holding, on the outer periphery of said second disc unit, asecond point which is diametrically opposite to said first point on saidsecond disc unit, above, on the outer periphery of said first disc unit,a second point which is diametrically opposite to said first point onsaid first disc unit; slowly lowering said second point on said seconddisc unit so that said first and second disc units are gradually broughtinto contact with each other, first at said first points and finally atsaid second points; and curing said adhesive.
 2. A method according toclaim 1, wherein said step of curing is carried out under normalatmospheric pressure.
 3. A method of manufacturing an optical disc, inwhich a first and a second disc unit, at least one of which includes arecording layer, are bonded together with an adhesive, said methodcomprising the steps of:disposing said first disc unit generallyhorizontally on a shaft; applying an adhesive onto said first disc unit;said adhesive being a room-temperature curing two-pack epoxy adhesivecomprising a bisphenol epoxy resin as a base agent and a modifiedaliphatic polyamine as a curing agent, said adhesive having a viscosityof 100-1000 cps, a pot life of one hour or more, a cure shrinkage of1.0% or less, and a water absorption after curing of 0.2% or less;fitting said second disc unit on said shaft, initially out of contactwith said first disc unit; bringing a first point on an outer peripheryof said second disc unit into contact with a first point on an outerperiphery of the first disc unit, whereby said second disc unit isplaced in a slanting position relative to said first disc unit; holding,on the outer periphery of said second disc unit, a second point which isdiametrically opposite to said first point on said second disc unit,above, on the outer periphery of said first disc unit, a second pointwhich is diametrically opposite to said first point on said first discunit, whereby said second disc unit is held slanting relative to saidfirst disc unit; lowering said second point on said second disc unit sothat said first and second disc units are gradually brought into contactwith each other, first at said first points and finally at said secondpoints; and curing said adhesive.
 4. A method according to claim 3,wherein said step of curing is carried out under normal atmosphericpressure.